1. Field of the Invention
This invention relates to pneumatic tires and has particular reference to such a tire constructed with reduced profile and with improved bondability between a cap tread and a sidewall.
2. Description of the Prior Art
When pneumatic tires are produced, a cap tread is brought into integral joining with a sidewall. To this end a "Side Over Cap (SOC)" structure is commonly accepted in which the cap tread is disposed to be overlapped at its two opposite sides with two corresponding sides of the sidewall.
To attain several important qualities of a tire of the character mentioned, a certain rubber composition has been used to form a cap tread, which composition is made up of a styrene-butadiene rubber of a high styrene content blended in an amount of more than 50 parts by weight per 100 parts by weight of the total rubbery component. Such a rubber, referred to simply as HS-SBR, is configured to have a styrene content of greater than 28% by weight and reputed as exhibiting sufficient resistance to wet skidding.
The above rubber composition, however, is rather poor in tackiness and hence less adhesive to a rubber material in common use for sidewall formation. This poses vulcanization failures, namely objectionable blisters or air entrainments which may take place at a shoulder portion of the cap tread after vulcanization of the resulting green tire. For convenience such nature of failure is hereunder called "shoulder blistering".
A "Cap Over Side (COS)" structure is also known in which a cap tread is joined with a sidewall with use of a tie rubber laid at a shoulder portion of the cap tread. The tie rubber is rendered similar to a rubber material employed for the sidewall. In that instance the tie rubber is positioned over two opposite sides of the cap tread after which the whole is extruded into an integral form, followed by assembling of the extrudate with the sidewall. The tie rubber-laden sides of the cap tread are held in overlapped relation to the sidewall.
The prior tie rubber generally of natural rubber, cis-1,4-butadiene rubber or their mixture is highly shrinkable with heat and necessarily shorter at both ends than the tread rubber while in vulcanization. This needs tedious forcible pulling of the ends of the tie rubber so as to effect joining with the sidewall.
In the case where it is applied to a pneumatic tire of a cross-sectionally low profile or height, say of 60% or below in aspect ratio, the COS structure tends to cause released or peeled joint between the cap tread and the sidewall under flexing load during running of the tire. In the COS structure the sidewall is smaller in length than the tie rubber-laden tread and thus liable to displace out of the overlapped joint or otherwise get abutted against or matched with a marginal edge of the tie rubber-laden tread.